It is believed that the response to Thy-1 antigen, and presumably to other alloantigens, is influenced by three phenomena: H-2 restriction similar to Doherty-Zinkernagel phenomenon, Ir control; and antigenic competition. One may speculate that these three phenomena are in some way interconnected and possibly may have a common denominator. To provide support for the above contention, the immune response of mice to the Thy-1 alloantigen will be studied. The response elicited in vivo or in vitro by Thy-1 antigens of thymocytes will be detected and measured by means of plaque assay and spot-test. These techniques detect antibody producing cells and allow the quantitation of serum antibodies. Major part of studies will be carried out using anti-Thy-1.1 response as an experimental model. The experimental work will be carried out in four major areas: (1) The recently found phenomenon of H-2 compatibility requirement for anti-Thy-1.1 response will be studied to define its mechanism and genetics, (2) The effect of non-H-2 alloantigens will be investigated to determine whether such alloantigens exert stimulatory or inhibitory influence on anti-Thy-1.1 response. Furthermore, attempts to characterize product and function of Ir-5 gene will be made. (3) To facilitate the studies and subsequent interpretation of the data gathered, cellular requirements for anti-Thy-1 response will be defined both in vivo and in vitro, and (4) Finally, in order to be able to draw more generalized conclusions from data accumulated in above mentioned areas, systematic studies of the anti-Thy-1.2 response will be carried out.